Product formation (stoichiometry) linked to microbial community
structure
The microbial community structure at 40-50 d SRT was dominated byBifidobacterium scardovii and supported by Megasphaera andPropionibacterium based on 16S rRNA analysis. This microbial
structure could explain the observed product spectrum of propionate and
acetate. Bifidobacterium are known for the production of acetic
and lactic acid using the phospho-ketolase pathway from glucose using
the ‘bifid shunt’, though no propionate production has been as yet
associated with Bifidobacterium (Falsen et al., 2015; Pokusaeva,
Fitzgerald, & Van Sinderen, 2011; Ventura, Delgado, Milani,
O’callaghan, & Van Sinderen, 2016). The microorganism is lacking the
key enzymes for the glycolysis and hexose-monophosphate pathway and uses
the phospho-ketolase pathway for catabolism (de Vries & Stouthamer,
1967). Possibly, the other observed species Megasphaera
cerevisiae and Propionibacterium thoenii capable of consuming
the lactate and producing propionate were responsible for the production
of propionate in this study (Lanjekar, Marathe, Ramana, Shouche, &
Ranade, 2014; Paikt & Glatz, 1997). Potentially, lactate consumption
was faster than lactate production at elevated SRT-values (10-20 d and
40-50 d) since no transient lactate was observed. Looking at the direct
fermentation of glucose to propionate and acetate a product spectrum of
2:1 (molpropionate:molacetate) can be
expected (Gonzalez-Garcia et al., 2017). In this study a product
spectrum at 40-50 d SRT of propionate:acetate of 2.05:1
(molpropionate:molacetate) was found,
this reflects nicely the proposed stoichiometry by Gonzalez-Garcia et
al., (2017). Bifidobacterium was producing lactate and acetate,
and subsequently a different microorganism could produce propionate and
acetate from lactate.
The microorganism that dominated the process shortly after startupClostridium pasteurianum was also encountered in (J. Tamis et
al., 2015). In that work C. pasteurianum dominated the granular
biomass over a pH range of 4.5-5.5. Two differences between that study
and this study for obtaining C. pasteurianum were that the
settling time was set to 2 min and the biogas was continuous diluted by
input of fresh N2. C. pasteurianum produced a
product spectrum dominated by acetate, butyrate and hydrogen as several
studies have reported before for the Clostridium genus
(Crabbendam, Neijssel, Tempest, & Amsterdam, 1985; Dabrock, Bahl, &
Gottschalk, 1992; Lin et al., 2007; J. Tamis et al., 2015). This study
shows that there is still a knowledge gap for obtaining specific product
spectrums using granular sludge for the production of VFA from a
carbohydrate-rich stream.